Automatic volume control with noise suppressor



Nov. 17, 1936.

G. L. BEERs 2,060,969

AUTOMATIC VOLUME CONTROL WITH NOISE SUPPRESSOR Filed March l5, 1953 2 Sheets-Sheet 1 INVENTOR GEORGE L. BEERS HEY ATTORNEY Nov. 17, 1936. G, BEERS l 2,060,969

AUTOMATIO VOLUME CONTROL WITH NOISE SUPPRESSOR Filed March l5, 1933 2 Sheets-Sheet 2 Riff/Nm ,wa/f ,am

afb/Mm? INVENTOR GEORGE L. EERs r [wm ATTORNEY Patented Nov. 17, 1936 UNITED STATES AUTOMATIC' VOLUME CONTROL WITH NOISE SUPPRESSOR George L. Beers, Collingswood, N. J., assigner to Radio Corporation of America, a corporation of Delaware Application March 15,

Claims.

My present invention relates to radio receiver control systems, and more particularly to an automatic volume control arrangement utilizing a multi-vibrator oscillator as a noise suppressor.

It may be stated that it is one of the prime objects oi my present invention to provide a suppressed background automatic volume control arrangement for a radio receiver wherein a multivibrator oscillator is employed in the suppressor circuit.

Another important object of the present invention is to provide in a radio receiver which includes an automatic volume control arrangement, a background noise suppressor device which comprises a multi-vibrator oscillator adapted to impair the operating eciency of the detector or audio frequency amplifier of the receiver when the signal input to the receiver decreases below a predetermined intensity level.

The novel features which I believe to be characteristic of my invention are set forth in particularity in the appended claims, the invention itself, however, as to both its organization and method of operation will best be understood by reference to the following description taken in connection with the drawings in which I have indicated diagrammatically two circuit organizations whereby my invention may be carried into effect.

In the drawings,

Fig. 1 shows a diagram of a receiver embodying the invention,

Fig. 2 shows a modification.

Referring now to the accompanying drawings, which diagrammatically illustrates in Fig. l a radio receiver embodying the present invention, it will be noted that the receiver includes a source of signals l, which is coupled, as at M, to the resonant input circuit of an amplifier tube S, preferably of the screen grid typ-e, and that the output of the amplifier is coupled, as at M1, to the resonant input circuit of the detector tube A. The audio frequency signal energy in the output circuit of the detector tube A is transmitted, 45 through a coupling M2, to the input electrodes of an audio frequency amplifier tube A1, and the amplified audio output of tube A1 is then impressed, through a coupling M3, upon any utilization means, such as another stage of audio fre- 5 quency amplification and/or a reproducer of any desired type.

The receiver embodying the elements l, S, A and A1 may be included in a radio receiver of the tuned radio frequency type, or in one of the superheterodyne type. If the receiver is of the 1933, Serial N0. 660,885

(Cl. Z-20) tuned radio frequency type, then the resonant input circuits of the amplifier S and the detector A would have their condensers 2 and 3 variable, and rotors uni-controlled for tuning these circuits in the well known fashion. Of course, the 5 source of signals could then include one, or more, stages of radio frequency amplification, and would be uni-controlled in tuning with the input circuits of tubes S and A.

On the other hand, if' the receiver is of the superheterodyne type, then the amplier tube S designates an intermediate frequency amplifier tube, the condensers 2 and 3 being fixed and resonating the input circuits of the tubes to the 0perating intermediate frequency. As is well 15 known, the anode circuit of the amplifier S would then include the condenser l to resonate it to the same intermediate frequency. The detector A would then function as the second detector, and the source of signals l would comprise the usual network preceding the intermediate frequency amplifier. Such a preceding network includes at least a stage of tuned radio frequency amplification ahead of a first detector, and a local oscillator coupled to the first detector, the output cir- 25 cuit of the first detector being coupled to the input electrodes of the amplifier S and providing the intermediate frequency energy;

To maintain the signal input to the detector tube A substantially constant over a wide range of signal input amplitude variations to the receiver, an automatic volume control arrangement is utilized. Such an arrangement utilizes the direct current component in the anode circuit of the detector A as a source of biasing potential for the amplifier S. Thus, the control electrode of the tube S is connected by a lead 5, designated additionally by the symbol A. V. C. to point out that this lead is the automatic volume control path, to a desired point on the resistor E. The biasing resistor 6 is connected between the anode of tube A and a point on the voltage divider, or potentiometer P, this point being positive with respect to the point on the divider P to which the cathode of the tube A is connected. The control grid of the detector tube A is connected to a point on the divider P which is negative with respect to point 1 to which the cathode is connected.

The divider P may be employed to provide the necessary energizing potentials for the electrodes of the amplifier tube S. Thus, the anode of tube S is connected through lead 8 to a point on the divider P which is positive with respect to the point 'I' to which the anode of tube A is connected; the cathode of the amplifier tube S is connected by a lead 9 to the point 8 on the divider. The operation of the automatic volume control arrangement described is believed too well known to warrant a lengthy explanation.

Briefly, when the amplified signal energy which is impressed upon the input circuit of the detector A increases above a desired level, then the grid of the tube S is biased negatively with respect to the cathode of tube S a sufficient amount to reduce the gain of the amplifier to such an extent that the energy impressed on the input of tube A is decreased to the aforementioned desired level. On the other hand, if the signal energy impressed upon the input circuit of the detector A decreases below this predetermined level, then the potential drop across the resistor 6 decreases suiiiciently to bias the grid of tube S `less negative with respect to the cathode, and this decrease in negative bias is suiiicient to increase the gain of the amplifier tube S to such an extent that the signal energy delivered to the input circuit of the detector is maintained at the desired operating level.

Thus, the audio frequency signal energy impressed upon the audio frequency amplifier` tube A1 through the audio frequency transformer M2, is maintained substantially constant regardless of variations in signal energy collected by the receiver. Manual control of the receiver output is provided by contact device 6 by which any portion of the audio frequency potential developed across resistor 6 may be'impressed on the coupling device M2.

The control electrode of the amplifier tube A1 is maintained at a negative potential with respect to its cathode by connecting the grid through resistor F to a point on the bias resistor P1 which this will be more fully described at a later point.

Since resistors P and P1 are connected'in series, a single source of potential may be connectedito the points marked and -lrespectively. The bias of rectifier tube E is normally adjusted so that in the absence of a signal impressed on its grid the current flowing through resistor F is very small, and the bias on tube A1 is then such that it amplifies in the normal manner.

In order to prevent the reproduction of undesirable background noise, when the receiver is tuned between stations, various devices may be employed. The drawings show one such device; such a device acts to bias the audio frequency amplier to cut-off when the signal energy impressed upon the detector input decreases belov7 the predetermined minimum signal level. The control of the amplifier A1 is accomplished ny means 'of the element F, and in this case is shown as a bias resistor, the potentialY drop across this resistor being regulated by means of the network includingv the amplifier tube B, the multi-vibrator oscillator comprising tubes C and D, and the rectifier tube E. This-background noise suppressor network will now be described in detail.

The'v tube B is employed to amplify thepotential drop across the resistor 6. The grid of tube B is connected to the anode of-tube A through a direct current connection. The cathode of the tube B is connected to a point I6 :on theY divider P which is positive with respect to' the point 1', and the anode of tube B` is directly connected to the grid of the multi-vibrator oscillator tube C. The point I6 on the divider P is positive with respect to the point I6 to which the cathode of tube B is connected, and this point I6 is connected to the anode and grid of tubes B and C respectively through a resistor I'I, while the point I6 is connected through a resistor Il to the grid of the other oscillator tube D. The point I8 on divider P which is positive with respect to point I6, is connected to the anode of the tube C through a path which includes the coil I 9 and the resistor 20 in series with each other, and this point i3 is similarly connected to the anode of oscillator tube D through a resistor 2I. B'oth cathodes of tubes C and D are connected to the point I9' Which is more positive than point I6 and less positive than point I8.

The anode of oscillator tube D is connected through a feedback condenser 22 to the grid of tube C, while the anode of the latter tube is connected to the grid of tube D through a feed-back condenser 22. The output of the multi-vibrator oscillator is impressed upon the input circuit of the rectifier tube E through a path which comprises a low or high pass filter with a sharp cutoff. The coil I9 in the plate circuit of tube C is coupled to the coil I9, the latter coil being connected across the input terminals of the filter 3D,

and the resistor 3|, connected across the output terminals of filter 30, is connected in the grid circuit of rectifier tube E.

Specifically, the filter 3D is shown as a high pass filter of a design well known to those skilled in the art. It will be noted that the grid of rectiner tube E is connected to a point on the divider P which is positive with respect to point I8; the cathode of tube E is connected to a still more positive point; while the anode of tube E is connected to the junction of P and P1 through resistor F. Resistors P and P1 may be constructed as a single resistor. By-pass condensers, 4I), 40 and 4D are employed respectively between the bias resistor F and the cathodes of tubes A1 and E, and between the anode and cathode of the detector tube A.

As stated heretofore the device which it is desired to control by the multi-vibrator oscillator network is the bias resistor F which is connected in the plate circuit of rectifier tube E. This device F may be a relay if desired, and in that case could be used to operate a reproducer short-circuiting device, as described in my co-pending application Serial No.1475,4'74, filed August l5, 1930.

The frequency of a multi-vibrator oscillator, of the type including the tubes C and D, is controlled by the incoming signal. The filter 3i] couples the oscillator to the rectifier, and by using a filter with a sharp cut-ofi characteristic a slight change in the frequency of the oscillator produced by the incoming signal will cause a large change in the oscillator voltage impressed on the grid of the rectifier E, and, therefore, a large change in the direct current component flowing in the plate circuit of tube E. It is this direct currentcomponent change which is utilized to produce the background noise suppression function.

It has been experimentally determined that the frequency of a multi-Vibrator oscillator can be varied through several hundred kilocycles by varying the. plat-e impedance of a tube. By the use of a suit-able rectifier connected so that the direct current component of the rectified voltage is used to supply bias for a second tube and thereby vary its plate impedance, the frequency of a multi-vibrator oscillator can be controlled by the amplitude of a received signal. By connecting either -a low or high pass filter ahead of another rectifier the plate current of this tube can be controlled by the amplitude of the incoming signal. If the filter ahead of this last mentioned rectier has a sharp cut-off the change in average plate current of the second rectifier, or control tube, will occur suddenly with a particular value of signal input. Since the steepness of the filter characteristic and the output of the multi-vibrator oscillator governs the amount of control which can be obtained from such an arrangement, it is possible to obtain sufcient plate current change in the control tube E to vary the bias of either the audio frequency amplifier, or the detector, so that they do not operate when signal energy below a predetermined level is being received.

Specifically considering the circuit elements of this arrangement, when a signal is impressed on the grid of tube A, the average plate current of this tube is increased since it is biased near its cut-off point. The negative bias on the tube B is, therefore, increased due to the potential drop across the resistor G in the plate circuit of tube A. The plate impedance of tube B, which shunts the resistor Il in the grid circuit of tube C is, therefore, increased, thus increasing the effective impedance in the grid circuit of oscillator tube C, thereby lowering the frequency of the multivibrator oscillator.

The multi-vibrator is a two stage resistance coupled amplifier in which the voltage developed in the output of the second tube is applied to the input of the first tube. Such an arrangement will oscillate because each tube produces a phase shift of degrees, causing the output of the second tube to supply an input voltage to the first tube that is in exactly the right phase to sustain oscillations. Since there is no resonant circuit, the generated frequency is unstable, and the wave from some of the oscillating currents is very distorted. It is because of these properties that the multi-vibrator finds its usefulness. It is believed that the present description of the structure of the multi-vibrator and its general method of operation is sufficient for the purposes of the present application. Reference is made to Radio Engineering by F. E. Terman, pages 273 to 276 inclusive for a more detailed theoretical explanation of the operation of the multi-vibrator oscillator.

The filter 39 between the multi-vibrator and the rectifier E is of the high pass type and is designed to provide a high attenuation for the frequency at which the multi-vibrator oscillator oscill-ates when a signal is being received. When the frequency of the oscillator is increased by tuning the receiver between stations, a large input voltage is impressed on the grid of tube E. The amplitude of this voltage depends entirely upon the output of the oscillator and the cut-off characteristic of the high pass filter. When the large radio frequency voltage is impressed on the grid of tube E, it causes a large increase in the average plate current of tube E, since this tube is normally biased near its cut-off point. This change in plate current is used to' operate the device F, which, -as has been stated heretofore, may be a relay or a bias resistor.

Since a tube capable of handling large plate currents may be used for the tube E, and since the multi-vibrator can be designed to provide a high voltage output, it will be possible to obtain a change in plate current of tube E of at least 10 milliamperes with a comparatively small radio frequency input. It is pointed out that the contact device lli may be utilized as a manually operable device for controlling the signal level at which background noise suppression is to occur.

While the automatic volume control arrangement employed has been shown to comprise the utilization of the direct current component of the detector output, it is pointed out that the tube A need not be the detector tube of the receiver. For example, the tube m-ay readily be a special automatic volume control tube, as described in an article by Beers and Carlson in the Proceedings of the Institute of Radio Engineers for March, 1929, pages 501-515. Also, the automatic volume control arrangement is not limited to the control of the gain of a single amplifier, but, in the case of a superheterodyne receiver, may be used to control the gain of the radio frequency and intermediate frequency amplifier stages simultaneously.

In Fig. 2 is shown a modification of the inven tion wherein the plate potentials for tubes B, C, D, and E are all derived from the common source P2. This combination of tubes in effect comprises a direct current amplifier. The operation is the same as in the case of Fig. l, and need not be described again, since it is believed the diagrammatic showing will be sufficient. It is to be noted that in a conventional direct current amplifier the anodes of all the tubes cannot be supplied from a single source since the plate of one tube is directly connected to the grid of the following tube, etc.

While I have indicated and described a system for carrying my invention into effect7 it will be apparent to one skilled in the art that my invention is by no means limited to the particular e organization shown and described, but that many modifications may be made Without departing from the scope of my invention Vas set forth in the appended claims.

What I claim is:

1. In combination, a radio frequency amplifier, a rectifier coupled thereto, a biasing path between an input electrode of said amplifier and the output circuit of said rectifier for automatically regulating the gain of said amplifier to maintain the energy input to said rectifier substantially constant, an oscillator, a second rectifier, a filter network coupling the oscillator output to the rectifier input, means for coupling the output circuit of the first rectifier to said oscillator, and means, responsive to variations in the output circuit of the second rectifier, for automatically rendering the reproduction of the audio modulations of the radio frequency energy impressed on said amplifier inefficient when the intensity level of said impressed energy decreases below a desired minimum level.

2. In combination with a signal amplifier, a detector, an audio frequency amplifier, and means for automatically maintaining the signal amplitude at the detector input substantially uniform, a network for automatically regulating the trans mission efficiency of said audio amplifier, said network comprising an oscillator having its input circuit coupled to the output circuit of said detector, the oscillator having a frequency characteristic which is variable and dependent upon the current fiow in the detector output circuit, a rectifier having its input circuit coupled to the output circuit of said oscillator through a network having a predetermined frequency response characteristic; and a direct current connection between the output circuit of said'rectier and an input electrode of said audio amplifier.

3. In combination with a signal amplifier, a detector, an audio frequency amplifier, and` means for automatically maintaining the signal amplitude at the detector input substantially uniform, a network for automatically regulating the transmission eiciency of said audio amplifier, said network comprising an oscillator having its input circuit coupled to the output circuit of said detector, the oscillator having a frequency characteristic which is variable and dependent upon the current fiow vin the detector output circuit, a rectifier having its input circuit coupled to the output circuit of said oscillator, and a direct current connection between the output circuit of said rectifier and an input electrode of said audio amplifier, and the coupling between said oscillator zurand said rectifier including a high pass filter with a sharp cut-off characteristic.

`work comprising an oscillator having its input circuit coupled 'to the outputcircuit of said detector, a rectifier having its input circuit coupled .tothe output circuit of said oscillator through a `network having a predetermined frequency response characteristic, and a direct current connection between the output circuit of said rectifier and an input electrode of said audio amplifier, said oscillator being of the multi-vibrator type and including a pair of resistance coupled tubes, the frequency of the oscillator varying inversely with the effective impedance in the input circuit of one of the oscillator tubes. v

5. In combination with a signal amplifier, a detector, an audio frequency amplier, and means for automatically maintaining the signal amplitude at the detector input substantially uniform, a network for automatically regulating the transmission efiiciency of said audioamplifier, said network comprising an oscillator having its input circuit coupled to the output circuit of said detector, the oscillator having a frequency characteristic which is Variable and dependent upon the current fiow in the detector output circuit, a rectifier having its input circuit coupled to the output circuit of said oscillator through avnetwork having a predetermined frequency response characteristic, and a direct current connection between the output circuit of said rectifier and an input electrode of said audio amplifier, the coupiing between said detector and the input of said .oscillator including an amplifier tube for amplifying the potential drop across an impedance in the detector output circuit, the anode of said last named amplifier being connected to the grid of said oscillator tube.

6. In combination with a signal amplifier, detector, and an audio frequency amplifier, a network for automaticallyregulatingthe transmission efficiency of said audio amplifier,saidnetwork comprising an oscillator having its input circuit coupled to the output circuit of said detector, arectifier having its input circuit coupled to the ouput circuit of said oscillator, and a direct current connection between the output circuit of said rectifier and an input electrode of said audio amplifier, andthe coupling between said oscillator and sharp cut-off characteristic, said filter being designed to iprovide a high attenuation for the frecluding a radio frequency amplifier, a detector" and an audio frequency utilization network, an

arrangement:v for-impairing the.. output of said audio frequency network when the signal input to the receiver decreases below a desired mini- `mum level, said arrangement including a multil vibrator oscillator having its input circuit couypledto the output circuit of the detector, the frequency of the oscillator being controlled by the incoming signal, a rectifier, a filter network coupling the output of said oscillator to the input ofi? said rectifier, said filter having a characteristic E such'thata'slight 'change in the frequency of said multi-'vibrator produced by the incoming signal will cause a large change in the oscillator voltagev on the grid of said rectifier and a large change in' the iiow of direct' current in the anode circuit of said rectifier, and means in the rectifier anode circuit, responsive to the variations in the fiow of direct'current in the rectifier anode circuit, ,q

for impairing `said audio frequency output.

8. In combination with a radio receiver including a radio frequency amplifier, a detector and an audio frequency utilization network, an arrangement forimpairing the output of said audio frequency network when the signal input to the receiver decreases below a desired minimum level, said arrangement including a multi-vibrator oscillator having its input circuit coupled to the output circuit of the detector, the frequency of the oscillator being controlled by the incoming signal, a rectifier, a filter network coupling the output of said oscillator to the input of said rectifier, said filter having a characteristic such that a slight change in the frequency of said multivibrator produced by the'incoming signal willv causea large change in the oscillator voltage on the grid of said rectifier and a large change in the fiow of direct current in the anode circuit of said rectifier, and means in the rectifier anode circuit, responsive to the variations in the flow'f connection between the rectifier and said ampli`i fier, and a frequency selective path interconnecting said oscillator and said rectifier.

10. In a radio receiver, an audio amplifier, a signal rectifier, means responsive to received signal amplitude varations for automatically main-.1

taining the signal amplitude at the rectifier input substantially uniform, an oscillator having a variable frequency characteristic connected to said rectifier in such a manner as to be responsive in frequency to variations in signal carrier intensity;y

fiuctuations, means for rectifying the oscillator output energy, a network having a predetermined frequency response characteristic coupling said oscillator to said rectifying means, and means for utilizing rectified oscillator energy to regulate the" efficiency ofsaid amplifier.

l1. In combination with an amplifier and a demodulator, an oscillator coupled to said demodulator to be responsive in frequency to variations in intensity of thesignal carrier impressedf';

on said demodulator, means having a frequency response characteristic related to said oscillator frequency for deriving a uni-directional voltage from the oscillator output energy, means for regulating the amplifier gain with said Voltage, and additional means, responsive to variations in the demodulated currents, to control the amplitude of the signal energy fed to said demodulator.

12. In an automatic volume control system for a radio receiver of the type which includes a signal amplifier, a signal demodulator, and a network for utilizing the demodulated signal energy, means responsive to variations in amplitude of received signal energy for automatically regulating the signal amplifier in a sense to maintain the signal energy input to the demodulator substantially constant in spite of said variations, an oscillator circuit having a characteristic such that the frequency of oscillations produced by it is dependent upon the direct current component of the demodulated signal energy, and means for automatically regulating the efficiency of said network in response to variations in the frequency of said oscillations.

13. In an automatic volume control system for a radio receiver of the type which includes a signal amplifier, a signal demodulator, and a network for utilizing the demodulated signal energy, means responsive to variations in amplitude of received signal energy for automatically regulating the signal amplifier in a sense to maintain the signal energy input to the demodulator substantially constant in spite of said variations, an oscillator circuit having a characteristic such that the frequency of oscillations produced by it is dependent upon the direct current component of the demodulated signal energy, and means for automatically regulating the emciency of said network in response to variations in the frequency of said oscillations, said last named means including a rectier whose input is coupled to the output of said oscillator.

14. In a radio receiver of the type including a signal carrier amplifier, a demodulator, an audio frequency utilization network, and means responsive to signal amplitude variations for automatically varying the amplifier gain in a sense to maintain the signal carrier amplitude at the demodulator input substantially uniform, a background noise suppressor system which comprises an oscillator circuit for producing local oscillations of a definite frequency, a rectier for deriving a direct current component from said oscillations, means for utilizing said component to control the transmission efficiency of said audio network, and means responsive to the direct current component of rectified signals for controlling the frequency of the output energy of said oscillator.

15. In an electrical wave transmission system including a wave transmission tube, a demodulator and a utilization network, means responsive to wave amplitude variations for automatically varying the wave transmission efficiency through said tube in a sense to maintain the wave amplitude substantially uniform at the demodulator input, an oscillation producing network conf 

